Hi! Ok, so I bought a blackberry curve off ebay and I've been on the phone with at&t all day long trying to figure out how to get it to work. I can receive and send text messages, as well as connect to the internet- but I can't send or receive phone calls. When I try to make a call it says: "Sorry, only emergency calls are allowed". I've also made sure that my SIM and blackberry are registered and activated with the at&t network. At&t can't come up with any solutions for me- has this happened to anyone else, and if so, how did you fix it??

Sounds like you have a restrictive IT Policy on the device which is limiting the phone use.
Check here on the BlackBerry and see if you find any references to IT Policy:
Options > Security Options > General Settings

An IT policy preventing using the phone functionality of the device is what causes it.

__________________The difference between stupidity and genius is that genius has its limits.When you take things for granted, the things you are granted, get taken. Even a mosquito doesn't get a pat on the back until it starts to work.Too many people miss the silver lining because they're expecting gold.[BES 5.0.3 / GroupWise 2012 HP2]

I have the same problem on a 8310. There is NO it policy listed in the security settings. I tried jl cmdr to reset to factory and it has not made a difference. I have done this procedure to a different blackberry before and it worked fine. Any suggestions?